IBB ChartIBB data by YCharts

October was the biotech sector's worst month in 2013.

"Worst" is relative in an otherwise unbelievable year for biotech stocks, but as you can see from the chart above, our little corner of the market was beat by the S&P 500 in October.

June was the only other month this year during which the S&P 500 outperformed the biotech sector, but the broader market just lost less. IBB Chart IBB data by YCharts

There are many reasons for the recent weakness in biotech stocks. As the year ends, some investors have definitely decided to lighten up on their winners.

Catalyst Pharmaceuticals ( CPRX) was the best-performing stock between January and the end of September. And here's what Catalyst looked like in October: ^NBI Chart ^NBI data by YCharts

I probably had something to do with the Catalyst sell off.

Acadia Pharmaceuticals ( ACAD - Get Report) was the second-best performing stock January through September. And in October: ACAD Chart ACAD data by YCharts

Celldex Therapeutics ( CLDX - Get Report): Third-best return January through September. Not in October: nd CLDX Chart CLDX data by YCharts

Picking up the pattern? Here's Biocryst Pharmaceutics ( BCRX - Get Report), fourth best until Octoer: ^NBI Chart ^NBI data by YCharts

-- Reported by Adam Feuerstein in Boston.

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Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.